YSM Issue 86.1

FEATURE GEOLOGY
Tearing At the Seams:
The Splitting of the Indo-Australian Tectonic Plate
BY JAKE ALLEN
On April 11, 2012 a giant earthquake and a massive aftershock rocked
the seafloor of the Indian Ocean off the coast of Indonesia. Not only
were the earthquakes some of the most powerful ever recorded, but they
also puzzled scientists. Massive slabs of crust slid as far as thirty meters,
creating tremors that could be felt in India, and twisted the bedrock with
such intensity that several new fault lines formed. But these earthquakes
were centered in the middle of a tectonic plate, far from any established
fault lines, which was highly unusual. Recent studies of the earthquakes
suggest an uncommon and significant explanation for this incident: the
splitting of one of Earth’s tectonic plates.
The vast majority of earthquakes are caused by the movement of
tectonic plates, pieces of Earth’s crust and upper mantle that fit together
like the pieces of a jigsaw puzzle. The plates are not static, and build
up immense amounts of strain at their borders as the mantle flowing
underneath propels them forward, making them catch on other plates.
When the plates finally break free, they release this energy in a matter of
minutes as powerful waves, generating an earthquake. Since the plates
only interact with each other at the borders between them, this is where
the vast majority of earthquakes occur.
However, the 8.6 and 8.2 magnitude April earthquakes were focused
in the center of the enormous Indo-Australian Plate, a point hundreds
of kilometers from the closest plate boundaries.
“These were the kind of events that made seismologists do a double-
A schematic of a strike-slip fault similar to those observed in
the Indian Ocean earthquakes last April. Large slabs of crust
slipped over 20 meters along several such faults, unleashing
massive amounts of energy. Courtesy of the Southern California
Earthquake Center at USC.
take,” said Maureen Long, an assistant professor in the Geology and
Geophysics Department at Yale University, in reference to the April
earthquakes. “If you had taken a poll of seismologists before these events
and said ‘Is this possible?,’ most seismologists would have told you no,
including me.” For incredibly powerful earthquakes like these to occur
so far out of the way, an unconventional explanation was necessary.
Scientists now agree that the unusual earthquakes are matched by
an equally unusual cause. In 1986, an article published in the journal
Techtonophysics observed “intense intraplate deformation” on the Indo-
The tectonic plates of Earth. These regions of the lithosphere
fit together like a jigsaw puzzle, constantly colliding and interacting
with each other as they drift. Courtesy of University of
Wisconsin Eau-Claire.
Australian Plate. The lithosphere in this region was being warped like
modeling clay in the same location where the 2012 earthquakes would
later occur. According to seismologists today, the deformation mentioned
by the paper’s authors is an active process that is gradually ripping
the tectonic plate in two. Eventually, this may create a localized boundary
on the Indo-Australian Plate. Creating two new plates would certainly
involve magnitudes of energy on par with those seen in April, but the
necessary strain needs to come from somewhere else.
A recent study by a team at the University of California Santa Cruz
concluded that the internal strain on the plate is the result of differences
in the movement of the Indian and Australian regions of the plate.
While the northwestern Indian section collides with the Eurasian plate,
the Australian border shoves into Sumatra to the northeast. Like pulling
on the opposite ends of a wishbone, these opposing forces subject the
center of the plate to great internal stress. When the crust reaches its
breaking point, an event like the one in April this year occurs.
“Given the way these things were moving, something had to be happening
between them,” said Long. “The earthquakes in April provided
some direct data confirming that this diffuse zone of deformation is in
the process of localizing into a new plate boundary.”
The severe strain in this diffuse deformation zone led to a peculiar
series of earthquakes. The Santa Cruz team found that the earthquakes
were caused by ruptures along four distinct faults whereas most earthquakes
involve just one. The faults were also incredibly deep, extending
far into the mantle, and were reported to slip 20–30 meters in just a
few minutes. Such a massive release of energy is seldom seen, even on
the most active plate boundaries. Only the enormous internal strain
caused by the splitting of a major plate could create extreme lithospheric
deformation and earthquakes unheard of miles from the nearest plate
boundary.
The earthquakes of April 2012 have been some of the most intensively
studied tectonic events and their significance has yet to be fully understood.
Such powerful ruptures far from plate boundaries have altered
the way seismologists think about the causes of earthquakes. Uncommon
earthquakes like these provide scientists with significant insights
into how our planet is shifting and changing, right underneath our feet.
26 Yale Scientific Magazine | January 2013 www.yalescientific.org